As photosensitive lithographic printing plates, there have been widely employed PS plates comprising a hydrophilic support and a lipophilic photosensitive resin layer formed thereon. As a plate-making method therefor, it has been a common practice to perform mask-exposure (face exposure) via a lith film followed by the dissolution and removal of non-image parts, thereby giving a desired printing plate.
As photosensitive lithographic printing plate precursors, there have been widely employed PS plates comprising a hydrophilic support and a lipophilic photosensitive resin layer formed thereon. As a plate-making method therefor, it has been a common practice to perform mask-exposure (face exposure) via a lith film followed by the dissolution and removal of non-image parts, thereby giving a desired printing plate. In recent years, digitization techniques whereby image data are electronically processed, stored and output with the use of a computer have been widely diffused. Thus, various image-outputting systems for these digitization techniques have been put into practical use. As a result, it has been urgently required to establish computer-to-plate (CTP) technology whereby an image is scanned with highly directional light such as laser beams in accordance with digitized data and thus a printing plate is directly made without resort to a lith film. Accordingly, it is an important technical problem to provide a lithographic printing plate precursor appropriate for the technology.
As such lithographic printing plate precursors usable in scanning exposure, there has been proposed a construction comprising a hydrophilic support and a lipophilic photosensitive resin layer (hereinafter also called a photosensitive layer) containing a photosensitive compound, which can generate an active species such as a radical or bronsted acid upon laser-exposure, formed on the support and such a product has been already put on the market. When a lithographic printing plate precursor of this type is laser-scanned in accordance with digital data, an active species thus generated induces a physical or chemical change in the photosensitive layer and insolubilizes the same. Next, the lithographic printing plate precursor is subjected to development, thereby giving a lithographic printing plate of the negative type. Among all, a lithographic printing plate precursor having a hydrophilic support and a photosensitive layer of the photopolymerization type, which is formed on the support and contains a photopolymerization initiator showing a favorable photosensitization speed, an ethylenically unsaturated compound capable of undergoing addition polymerization and a binder polymer soluble in alkali developing solution, together with, if needed, an oxygen-blocking protective layer has advantages such as a high productivity, convenience in development, high resolution and favorable ink impression and, therefore, available as a printing plate having desirable printing performance.
To improve press life of printing plates, use has been made of urethane resin binders (see, for example, JP-B-7-120040, JP-B-7-120041, JP-B-7-120042, JP-B-8-12424, JP-A-63-287944, JP-A-63-287947 and JP-A-1-271741). In these documents, however, no description is made on image drawing by laser-scanning exposure. Although there are lithographic printing plates which contain a urethane binder and are suitable for laser-scanning exposure, these plates are still insufficient in press life and productivity. That is to say, as scanning speed is elevated to further improve productivity, exposure energy per unit area is lowered and thus the discrimination of exposed part/unexposed part is lessened. As a result, there arises a problem that the image part is frequently damaged by an alkali component in the developing solution and thus a favorable press life cannot be obtained.